The cleanliness of electronic circuit assemblies (ECA), such as printed circuit boards (PCB) or printed wiring boards (PWB), is generally regarded as being critical to their functional reliability. Ionic and nonionic contamination on circuit assemblies is believed to contribute to premature failures of the circuit assemblies by allowing short circuits to develop.
In the manufacture of electronic circuit assemblies, ionic and nonionic contamination can accumulate after one or more steps of the process. Circuit assembly materials are plated, etched, handled by operators in assembly, coated with corrosive or potentially corrosive fluxes and finally soldered.
In the fabrication of electronic circuit assemblies, e.g., printed circuit boards, soldering fluxes are first applied to the substrate board material to ensure firm, uniform bonding of the solder. These soldering fluxes fall into two broad categories: rosin and non-rosin, or water soluble, fluxes. The rosin fluxes, which are generally only moderately corrosive and have a much longer history of use, are still widely used throughout the electronics industry. The water soluble fluxes, which are a more recent development, are being used increasingly in consumer products applications. Because water soluble fluxes contain strong acids and/or amine hydrohalides, such fluxes are very corrosive. Unfortunately, residues of any flux can cause circuit failure if residual traces of the material are not carefully removed following soldering and thus remain on an electronic circuit assembly.
While water soluble fluxes can be easily removed with warm, soapy water, the removal of rosin flux from printed circuit boards is more difficult and has therefore traditionally been carried out with the use of chlorinated hydrocarbon solvents such as 1,1,1,-trichlorethane, trichloroethylene, trichloromonofluoromethane, methylene chloride, trichlorotrifluoroethane (CFC113), tetrachlorodifluoroethane (CFC112) or mixtures or azeotropes of these and/or other solvents. These solvents are undesirable, however, because they are toxic and when released into the environment deplete the ozone layer and/or contribute to the greenhouse global warming effect. Thus, use of such solvents is subject to close scrutiny by the Occupational Safety and Health Administration (OSHA) and the Environmental Protection Agency (EPA) and stringent containment equipment must be used. Moreover, if released into the environment these solvents are not readily biodegradable and are thus hazardous for long periods of time.
Alkaline cleaning compounds known as the alkanolamines, usually in the form of monoethanolamine, have been used for rosin flux removal as an alternative to the toxic chlorinated hydrocarbon solvents. These high pH compounds (e.g., about 12 pH), chemically react with rosin flux to form a rosin soap through the process of saponification. Unfortunately, these compounds, as well as the water soluble soldering fluxes, have a tendency to cause corrosion on the surfaces and interfaces of printed wiring boards if such compounds and fluxes are not completely and rapidly removed during the fabrication process.
Other approaches have been used including a highly caustic solution having a pH of 13 in a batch cleaning process and use of terpene compounds in combination with terpene emulsifying surfactants.
The complete removal of adhesive and other residues also poses a problem. During the manufacture of electronic circuit assemblies the components are mounted on the upper surface of the board with leads protruding downwardly through holes in the board and are secured to the bottom surface of the board by means of an adhesive. Further, it is sometimes necessary to temporarily protect certain portions of the board from processing steps such as the process of creating corrosion resistant gold connecting tabs at the board edges. This transient protection of portions of the circuit board can be achieved by the application of special adhesive tape to susceptible areas. Once such protection is no longer needed, the adhesive tape must be removed. In both instances, a residue of adhesive generally remains which, if not thoroughly removed, can cause premature board failure. Removal of this adhesive residue has traditionally been carried out by the use of chlorinated solvents which, as already described, are toxic and environmentally undesirable.
Thus, the residual contaminants which are likely to be found on electronic circuit assemblies and which must be removed include, but are not limited to, for example, rosin flux, photoresist, solder masks, adhesives, machine oils, greases, silicones, lanolin, mold release, polyglycols and plasticizers.
Instead of the toxic chlorinated hydrocarbon solvents and the highly corrosive and highly basic amine cleaning solutions and the other organic solvent cleaning systems which pose problems to the environment, an alternative aqueous alkaline cleaning solution has been developed by the assignee to remove the rosin fluxes, adhesive residues and the like from electronic circuit assemblies. These compositions are set forth in copending commonly assigned U.S. Patent Application Nos. 896,381; 896,660; 896,409; 896,365; 896,379, all filed Jun. 10, 1992 and 731,512, filed Jul. 17, 1991. These applications disclose aqueous alkaline cleaning solutions containing alkaline salts such as alkali metal carbonates and mixtures thereof with alkali metal bicarbonates and adjuvants such as alkali metal silicates to inhibit corrosion and brighten metal components, surfactants to aid in the cleaning efficacy of the cleaning solution, hydrotropes to maintain the surfactant in solution and stabilizers to maintain the alkali metal silicate in solution. These cleaning solutions have a pH ranging from about 10 to 13 and, in particular, are effective at solution pH's of less than 12 and are preferably utilized with an adequate reserve of titratable alkalinity to provide effective cleaning of the rosin fluxes and other adhesive residues and the like from the circuit assemblies.
For the removal of rosin soldering flux deposits, adhesives and the other residues during printed circuit/wiring board fabrication, the aqueous alkaline compositions are applied to the boards by immersion in dip tanks or by hand or mechanical brushing. Alternatively, the cleaning solutions can be applied by any of the commercially available printed wiring board cleaning equipment. Dishwasher size units may be employed, or much larger cleaning systems such as the "Poly-Clean +" and the various "Hydro-Station" models produced by Hollis Automation, Inc. of Nashua, N.H.
The temperature at which the flux removing compositions are applied typically range from room, or ambient, temperature (about 70.degree. F.) to about 180.degree. F., preferably, about 140.degree. to 170.degree. F. The flux removing compositions or concentrates are diluted with water to from as low as about 0.1 percent by weight (or volume) concentration and up to about 15 percent by weight.
Once the solder flux has been loosened and removed during a period of contact which typically ranges from about 1 to about 5 minutes, but may be longer up to about 10 minutes, the boards are taken from the flux removing solution. Another advantage of the aqueous alkaline cleaning solutions described previously and in the aforementioned copending assigned U.S. Applications is that these flux removing solutions need not be flushed with solvents as with the processes of the prior art. The boards are simply flushed with water for a period of up to about 2 minutes. Deionized water is preferred. The optimal rinsing time varies according to the kinds of surfactants and the concentrations of the flux removing solutions used and can easily be determined by routine experimentation.
The effluents from both the wash and rinse cycles from the cleaning process contain rosin flux residues, lead, tin and other materials removed from the electronic circuit assemblies as well as alkali metal carbonate, bicarbonate and silicate salts in addition to surfactants and other polymeric stabilizers used in the cleaning compositions. These effluents without treatment cannot be sewered and, further, cannot be recycled for use in the wash and rinse cycles. For example, in the disposal of industrial cleaning solutions, municipalities often mandate the levels of impurities which are allowable in sewage effluents. Thus, the regulations frequently mandate maximum permitted pH, maximum allowable heavy metals and maximum allowable organics measured as BOD or COD.
Accordingly, a treatment process is needed to remove the contaminants from the wash and rinse water effluents and preferably, provide sufficient removal of these materials such that the water can be directly sewered.
In copending, commonly assigned U.S. application Ser. No. 08/085,005, filed Jun 29, 1993, a multistage process for treating aqueous alkaline effluents containing rosin fluxes, carbonates, heavy metals and the like is disclosed. The treatment process includes carbon absorption to remove organic residues, ion exchange to remove metals and aeration to remove dissolved CO.sub.2 formed from the carbonates during ion exchange. While the multistage process is an effective method for removing these contaminants from aqueous effluents, when the effluents are more highly concentrated in organic matter, carbonates, metal impurities and the like, passage of such effluents through carbon or ion exchange resin beds may soon overload such beds reducing the efficiency of contaminant removal and requiring substantial time and capital expense in regenerating the beds.